During papermaking operations, an aqueous slurry of cellulosic fibers known as "furnish" is dewatered on a moving screen or "wire" (sometimes referred to as a forming fabric). Fibers and other particulates become trapped on the wire, forming a fibrous web which is further processed into the end product. Depending on the grade of paper being produced, the furnish may also contain suspended solids such as mineral fillers and emulsified additives along with various soluble materials to improve and/or modify the paper properties.
Examples of such suspended solids include clay, calcium carbonate, titanium dioxide, dyes and rosin. To achieve the desired paper properties of various paper and paperboard grades, it is essential that these ingredients be retained efficiently during the forming of the paper web and not flow through the wire with the water draining from the web. Papermakers employ combinations of various additives, including very high mass acrylamide copolymers (retention aids), highly cationic polymers (promoters), and anionic colloidal particles or polymers (microparticles) to promote retention of these suspended solids.
The furnish used by most papermakers is composed mainly of cellulosic fibers such as groundwood pulp, chemithermomechanical pulp (CTMP), kraft pulp, or sulfite pulp.
It is the nature of these pulp varieties that they have, at the fiber surfaces, a negative (anionic) colloidal charge. Other pulp varieties or samples of furnish selected from some papermaking processes, however, may exhibit positive (cationic) colloidal charges.
Variations in the electrokinetic properties of the furnish, including the colloidal charge of the furnish, can have a profound effect on retention, drainage during web formation, and paper properties. For example, if a new batch of pulp contains a higher level of dissolved anionic polymers and colloidal anionic material (disco), then the percentage of fine material retained during a single pass over the forming fabric (first-pass retention) may be decreased. Variations in parameters such as retention and drainage can also have an immediate effect on the tension control of the machine, which effects dimensional stability and can lead to web breaks and attendant downtime as well as nonuniform web properties. Therefore, stability in the colloidal charge is a critical papermaking furnish characteristic since it can significantly affect the quality, uniformity and rate of production in papermaking.
Prior attempts to monitor and control the electrokinetic properties of papermaking furnish have included laboratory-based tests where a sample is withdrawn from the furnish or from the whitewater which drains from the furnish during web formation. Some on-line systems have also been attempted.
Lab-scale tests include micro-electrophoresis, in which a microscope is used to observe the motion of a particle in a capillary cell while under the influence of an imposed electric field. The quotient of the particle velocity and the field strength defines the particle's electrophoretic mobility. When the viscosity of the fluid and its conductivity are known, this method is used to calculate the "zeta potential", which is the electrical potential at the hydrodynamic slip plane at the surface of the particle. As a particle moves through the fluid, a double layer of charge is built up--a layer at the surface of the particle and another layer produced by diffuse ions that remain close to the particle's surface during motion. Zeta potential, which is the electrical potential created by both layers of charge, is frequently used by papermakers as an indication of the state of electrokinetic charge in the system.
In order for micro-electrophoresis to work, the particles must be much smaller than the diameter of the capillary cell in which the tests are conducted. This poses a significant limitation in furnish analysis due to the relatively large size of cellulose fibers used in papermaking. In many cases, this size limitation requires screening the papermaking furnish and testing only the fine suspended solids remaining in the filtrate, or white-water. The capillaries used are also subject to contamination by particulates within the furnish.
Moreover, focusing a microscope tends to be too delicate of an operation for practical application in the industrial environment of a paper mill. If the microscope is incorrectly focused, this can cause incorrect readings. In addition, the information obtained about the electrophoretic mobility of a particle is usually not linearly proportional with any process or additive which is under the direct control of papermakers.
Another approach has been to employ a streaming current apparatus to measure another electrokinetic effect related to the charged properties of polymers, colloids, and fine particles in the sample. A streaming current device typically employs a plunger or piston which is reciprocated within a polytetrafluoroethylene (PTFE, commonly referred to as Teflon.TM.) cylinder closed at one end. The diameter of the piston is slightly smaller than the diameter of the PTFE cylinder so that the reciprocating motion of the piston causes fluid to move rapidly in the narrow annular space between the piston and cylinder wall. The motion of the fluid through this space in the direction parallel to the cylinder movement induces a streaming current resulting from colloidal materials adsorbing onto the PTFE cylinder wall, which is detectable by appropriate connection of an ammeter or voltmeter to the cylinder wall. In use of a streaming current device, the papermaker may titrate a known volume of whitewater or furnish with a highly charged (cationic) polymer, such as poly-diallyldimethylammonium chloride (DADMAC), or other material. The titration is continued until the streaming current apparatus indicates a zero signal. The amount of titrant added is used to calculate the net charge of the sample. A significant disadvantage of the streaming current method, however, is that the method is significantly affected by materials such as sodium sulfate and potassium chloride which are commonly present in papermaking samples. Such materials tend to increase the conductivity of the sample and significantly reduce the ability to accurately measure electrokinetic properties of the furnish. Also, such devices do not exhibit equivalent utility for different types of furnish. For example, streaming current devices have been observed to perform poorly when carrying out a charge titration experiment with alkaline and de-inked pulps.
A further approach to determining the electrokinetic properties of fiber furnish is called streaming potential. This approach measures the potential resulting from fluid flow through a pad of fibers. As furnish flows through the pad, diffuse ions in the outer layer of charge are removed, allowing estimation of the zeta potential at the fiber surfaces. This potential is measured by placing electrodes adjacent the pad. The measurements are repeated often enough to give the papermaker "a good idea" of how the zeta potential at the fiber surface varies as a function of time.
Prior art streaming potential devices suffer from various disadvantages which tend to produce unreliable data. For example, the absolute magnitudes of the zeta potential determined from known streaming potential devices depends on the extent of compaction of the fiber pad, which is difficult to control. Furthermore, in order to obtain an accurate measure of the zeta potential of a sample it is necessary to repeat the experiment several times at different densities of the fiber mat and extrapolate to zero solids. Spurious results are often observed when measuring high conductivity pulps. Another difficulty is that the electrodes of prior art streaming potential devices tend to drift, producing inaccuracies in the measurements. Additionally, streaming potential measurement devices do not provide the papermaker with information about colloidal charge.
Therefore, it is an object of the present invention to provide a method and apparatus for determining an electrokinetic property of a papermaking furnish in order to provide reliable formation-related data to papermakers.
Another object of the present invention is to provide a method and apparatus for quantifying the colloidal charge of a papermaking furnish.
A further object of the present invention is to provide a method and apparatus for accurately and efficiently determining the algebraic sign and relative magnitude of streaming potential of a papermaking furnish to obtain superior control of charge balance within the furnish.
Yet another object of the present invention is to provide a method and apparatus of the character described which promotes efficient use of fiber furnish additives such as highly charged cationic polymers.
An additional object of the present invention is to provide an automated method and apparatus for determining an electrokinetic property of a papermaking furnish, and from that determination, adjusting furnish additives to achieve desired formation properties.
A still further object of the present invention is to provide a method and apparatus for determining an electrokinetic property of a papermaking furnish by repeatedly measuring streaming potential of a single furnish sample.
An additional object of the present invention is to provide a method and apparatus of the character described which promotes desired web formation properties through control of papermaking furnish composition based on a measured electrokinetic property of the furnish.
Another object of the present invention is to provide a method and apparatus for accurately predicting the proportional amounts of charged additives required to optimize paper machine performance, including retention, drainage, and minimization of property variations.
Yet another object of the present invention is to provide a method and apparatus which obtains accurate information about streaming potential through a fibrous pad without the need to measure or apply specific pressure differentials across the pad.